This is going to be a tough question so I don't expect many replys but it's something that most would like to know. Even as a mechanic, I can't be sure if I'm even correct or not.

The purpose of the proportioning valve (differential switch to some) is to control the amount of front to rear braking between front disk and rear drum brakes. Since the front disks have the weight transfer on them the valve allows more pressure to the front to keep the rear brakes from locking up too soon. Correct?

Older vehicles that had 4 wheel drum brakes didn't have a proportioning valve but then the older ones also only had a single master cylinder.

The big question:

Since my car has 4 wheel disk brakes, does it still require the proportioning valve. Does the valve limit brake pressure to the front or rear or is it just a pressure switch to let you know when pressure drops and turn on the light? And also is it just a junction block to allow the 2 lines from the master cylinder to go to the brakes as 3 lines?

The reason I'm asking is that I just swapped in a manual master cylinder into my car. Found it in a mid 80's S10. It has standard fittings and all the other fittings in my car are those metric bubble flare fittings. I cut off the fittings on the metric lines to the master cylinder and installed standard ends with double flares on them but I'm not sure if they'll leak yet or not. If I can eliminate the proportioning valve entirely and run standard lines throughout the car with a Tee fitting for the front brakes, it would solve a lot of problems.


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87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
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87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

This is straight out of my 89 GM shop manual, and hopefully it will help in this case:

The metering or hold off section of the combination valve limits pressure to the front disc brakes until a predetermined front pressure is reached, approximating the pressure to overcome the rear shoe and lining retractor springs. There is no restriction at inlet pressures below 20kPa (3 PSI) to allow for the pressure to be equalized when the brakes are not applied.

The proportioning section of the combination valve proportions oulet pressure to the rear brakes after a predetermined rear input pressure has been reached.

The valve has a by-pass feature which insures full system pressure to the rear brakes in the event of a front brake system failure. Similiarly, full front pressure is retained in the event of a rear brake system failure.

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1989 IROC-Z 5.7L

 

Omar, isn't the by-pass feature built into the master cylinder? I didn't think it was part of the prop valve.

Stephen, I was told what you're asking- that the prop valve for a 4-wheel-disc car doesn't do any metering. It's supposedly just a branch for splitting the m/c lines (2 lines to 3) and a home for the BRAKE pressure warning light. There IS, though, a valve on it, covered by a pipe plug, with a piece of rubber thru the center of the plug. I'm not sure what it is- I mistakenly disassembled it, and found a valve and spring inside. (Then I put it back together, and caused an air leak, and had to find a gasket sealer that was impervious to brake fluid, and ... ugh- don't take it apart. Here's my old message on that hell if you're interested: https://www.thirdgen.org/messgboard/...ML/000598.html ) So there is some kind of valving in there... but I don't know what it's for.

In fact, disc/drum prop valves have a rubber-cap-covered valve on the back (points towards the firewall). With the cap off, It looks like a tire pressure valve. It's meant to equalize pressure inside the valve when certain kinds of brake work are done. The disc/disc prop valves don't have any rubber-covered valve.. in fact there's no valve anywhere on 'em.

Ah- I found an old message on this: https://www.thirdgen.org/messgboard/...ML/000602.html


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!

 

Correct me if I'm wrong, too, but weren't there 2 different combo valves used in different years? Something like the early rear discs needed no proportioning but the later ones used the same combo valve as the drums?

 

Whether or not you need a prop valve depends on the caliper piston sizes and brake rotor diameter. When braking the weight transfers forward thereby requiring more front braking (as stated). Also, increasing traction by any method will increase forward weight transfer and a lower CG will decrease it.
One way to control proportioning is via different caliper piston sizes. A smaller caliper piston will exert more pressure on the rotor compared to a larger one.
Another way is with larger rotors. A larger rotor will provide more surface area/time than a smaller one. The rotor diameter is relative to the outside diameter of the tire, so putting larger rear tires, will decrease the amount of rear braking.
There are different types of proportioning valves; pressure, inertia and ride height.
The camaros tend to have too little rear brake bias IMHO. It is safe this way which is probably why it is so. Too much rear brake and the car will be really loose. By using an adjustable valve you can tailor it to your own preference. I install the adjustable valve in the rear brake line and run the fronts straight off the MC...standard procedure. BTW, I have a stock one which is OK since I dont race this car.

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1984 Z28 (L69) monochrome tangerine orange with IROC decals and hood
1987 IROC wheels with Goodyear GSC's
305 from 87 IROC with 1994 LT1 cam (203/208 .450/.460")
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Qjet is in the works
dynomax cat-back
non WC T-5 with 0.73 5th
3.27 nine-bolt

 

TomP: Mine is factory disk and I have that rubber plug covering somthing on the back towards the firewall.

Apeiron: I've never seen an early rear disk brake car. The early disk/drum valves look like they are made from cast iron with standard double flares. My 87 has an aluminum valve and those #&!* metric bubble flares.

I'm considering removing the combination valve and just installing the adjustable type valve for the rear brakes. I'll have to see how well my double flare on the metric lines works first. If I ever get around to installing a line lock then the metric lines will have to go anyway.

 

Stephen, my car has the early disc setup, I'll bring it over this weekend if you want to have a look. I'm just finishing up with the flu at work in Edmonton, going to be back in Calgary on Friday night.

 

Stephen, thanks for the info! That's weird! I put an '84/posi/3.73/disc axle into my car. I used the '84 prop valve. My '86 J50 (disc/drum) prop valve had the rubber-covered valve, but the '84 J65 (4-wheel-disc) prop valve didn't.

I wonder if that rubber-covered valve's a '85-up thing?

[edit] Oh- the '84 prop valve used the metric bubble flares, by the way.

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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!

[This message has been edited by TomP (edited March 07, 2001).]